Mission to touch the sun

The sun is the most familiar object in the heavens, the subject of centuries of casual observations and decades of scientific study, yet it presents something of a mystery. Temperatures on its visible surface, the photosphere, are about 5,500 Celsius. Inside, where fusion reactions generate unimaginable amounts of energy, temperatures reach a hellish 15 million degrees. It might seem obvious that the corona, the star's wispy atmosphere which is visible from Earth only during a solar eclipse, would be progressively cooler than the surface.

But it isn’t. Temperatures actually rise back up into the million-degree range over a vast volume of space surrounding the sun. They reach about two million Celsius at an altitude of six or eight million km, a distance equivalent to about ten solar radii above the surface. Here the charged particles that make up the solar wind undergo a bizarre acceleration, propelled by powerful magnetic fields which increase their speed by a factor of five.

The precise location and nature of these heating and acceleration effects are still something of a mystery, with rival scientific theories offering alternative explanations and predictions. Now those theories are to be put to the test by the Parker Solar Probe (PSP), which Nasa describes as a mission to touch the sun.

PSP launches in August 2018 (or possibly at the very end of July) aboard a Delta IV Heavy rocket from Cape Canaveral, setting out on a seven-year journey to make 24 orbits of the sun. On the way, it will make seven flybys of Venus, using the planet's gravity to boost it further inwards for its expedition to the most hostile environment in the solar system.

It will travel eight times closer to the sun than any previous spacecraft, and become the fastest human-made object in history as it hurtles around it. The probe's delicate instruments will be protected from roasting by a huge 11.4cm-thick carbon composite heatshield, maintaining the scientific payload at a temperature that would be comfortable in an office or laboratory on Earth.

PSP sets another record too: it is the first Nasa spacecraft to be named after a living person. The agency announced in May 2017 that the probe previously known as the Solar Probe Plus would now be called the Parker Solar Probe, in honour of the eminent space scientist Professor Eugene Parker. His 1958 prediction of the existence of the solar wind was initially greeted with scepticism, and his scientific paper on topic was even rejected by a couple of referees, before being confirmed by observations a few years later.

Professor Parker quite literally wrote the book on the solar wind, and will be 97 when his namesake spacecraft makes its first close approach to the sun in December 2024. Among his key predictions was that the charged particles and plasma streaming from the sun would be accelerated to supersonic velocities as they expanded into space.

Supersonic in a vacuum

But wait a second – how can there be a speed of sound in the silent vacuum of space?

We asked the head of the heliospheric physics laboratory at Nasa's Goddard Space Flight Center, Dr Adam Szabo, for clarification.

"The term is really a generalisation in the solar wind", Dr Szabo told All About Space. "Although there are sonic waves, 'supersonic' really refers to a bulk flow speed faster than any waves – including the fastest electromagnetic waves – can travel in the medium".

Beyond the supersonic threshold, information cannot flow back towards the sun but can only flow outward. This 'information' could relate to an obstacle or event within the flow, rather like the way traffic congestion on a motorway can be detected far upstream of any actual blockage.

In the inner part of the corona, the solar wind is expected to remain 'coupled' to the sun and rotating in time with it. Further out it moves radially, directly away from the star. Somewhere between these two conditions is the transition zone where the solar wind is given a huge burst of acceleration.

"We expect the solar wind to be as slow as 200 to 300 km/s (450,000-600,000mph)" said Dr Szabo, "which is about half what it is at 1 AU [at Earth's orbit]. But more importantly, the speed of the waves will go up with the higher densities and stronger magnetic fields.

"Models disagree on exactly where we should expect this transition, and the predictions range between ten and twenty solar radii.

"By going inside 10 Rs, we should see it – unless all the predictions are wrong". The closest approach PSP will make is 6.3 million km (3.9 million miles), or about nine solar radii.

Surfing the waves

Just as the behaviour of the corona defies common logic, the waves in the solar wind are also pretty bizarre. We're used to light waves with frequencies of a few hundred terahertz, and sound waves at frequencies of a few thousand hertz. The waves that form in plasma have names that could be straight out of Star Trek: they are known as magnetohydrodynamic waves or Alfvén waves, after the Swedish physicist Hannes Alfvén who first described them in 1942. Alfvén was another pioneering space scientist whose work was initially dismissed by more established rivals, and it was not until 1970 that he finally won recognition with a Nobel Prize.

The properties of Alfvén waves are strange indeed. Instead of having frequencies measured in cycles per second, an Alfvén oscillates just once in about four hours, at something like 0.00007Hz. The wavelength is correspondingly long, at around 120,000km or ten times the Earth's diameter.

PSP's mission is to detect and record these waves, deploying four main experiments in its search for the transition zone.

The Solar Wind Electrons Alphas and Protons (SWEAP) investigation will count electrons, protons and helium ions in the solar wind, actually catching some of them for direct analysis on board the probe. The acceleration of these particles and ions will be measured by the experiment called the Integrated Science Investigation of the Sun, or ISIS.

3D images of the corona, the solar wind, and shocks as they pass the spacecraft will be made by the Wide-field Imager for Solar Probe (WISPR), while the Fields Experiment will make direct measurements of electric, magnetic, and radio emissions, as well as measuring dust by recording voltage spikes when specks of matter hit the antenna.

These sensitive pieces of equipment have to go where no scientific instrument has gone before, and consequently the PSP employs radical innovations to protect them.

Most obvious is the 2.4m-diameter carbon composite heat shield or Thermal Protection System (TPS), which acts like a parasol to shade the spacecraft from the inferno. Only the particle detector, antennae, and the edges of the solar power generators peek around the edge of the TPS – the front face of which will reach nearly 1400C, hot enough to melt steel.

The solar array receives 25 times as much energy at close approach to the sun as it would at Earth, and to keep it operating efficiently an additional cooling system is used. Surprisingly, the coolant used is just five litres of pressurised water. This provides 6000 Watts of cooling power, which would be enough to cool a typical living room on Earth. In the corona, it will keep the solar arrays at a working temperature below 160C.

Fast and hot

As the Parker Solar Probe sweeps around the deep gravity well of the sun, it will be accelerated to speeds no other spacecraft has reached. Although the Juno probe to Jupiter was widely reported in 2016 as being the fastest human-made object, this was due to a misunderstanding of the measurement frame. Juno hit nearly 60 km/s (133,000mph), measured with respect to the sun, which is certainly shifting. By comparison, Voyager 1 practically trundles along at a mere 17 km/s (38,500mph).

There are actually two spacecraft that currently vie for the speed record title. Helios 2 was recorded at 98.9 km/s (221,000mph) in January 1989, and is recognised by Guinness World Records as the fastest ever. But some argue that, by the time it reached this speed, Helios 2 was a dead hulk and so no longer really a spacecraft. For them, sister ship Helios 1 remains champion, hitting 59.3 km/s (132,000mph) in February 1975.

The arguments will become redundant before long, as the Parker Solar Probe will comprehensively shatter the record. Sweeping around the sun, it will accelerate to an astonishing 200 km/s (450,000mph).

The Parker Solar Probe will launch during a 20-day window beginning on 31 July 2018, using one of the most powerful rockets in Nasa's arsenal. The Delta IV in 'heavy' configuration will include an integrated third stage booster, powered by the workhorse Star 48BV rocket motor that has been used on over 130 missions since the 1980s. This is necessary because of the substantial 610kg mass of the spacecraft, and the requirement for a big initial push to its first encounter with Venus just two months after launch.